[RFC PATCH 04/56] x86/bugs: Reset spectre_v1 mitigations

David Kaplan
2025-10-13T14:35:28Z

Add function to reset spectre_v1 mitigations back to their boot-time
defaults.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
arch/x86/include/asm/bugs.h | 1 +
arch/x86/kernel/cpu/bugs.c | 18 ++++++++++++++++++
2 files changed, 19 insertions(+)

diff --git a/arch/x86/include/asm/bugs.h b/arch/x86/include/asm/bugs.h
index f25ca2d709d4..e43b9412645e 100644
--- a/arch/x86/include/asm/bugs.h
+++ b/arch/x86/include/asm/bugs.h
@@ -11,5 +11,6 @@ static inline int ppro_with_ram_bug(void) { return 0; }
#endif

extern void cpu_bugs_smt_update(void);
+void arch_cpu_reset_mitigations(void);

#endif /* _ASM_X86_BUGS_H */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6a526ae1fe99..9d5c6a3e50e1 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -46,6 +46,8 @@
* may want to change based on other choices
* made. This function is optional.
* <vuln>_apply_mitigation() -- Enable the selected mitigation.
+ * <vuln>_reset_mitigation() -- Undo's the apply_mitigation step, this is used
+ * with runtime mitigation patching.
*
* The compile-time mitigation in all cases should be AUTO. An explicit
* command-line option can override AUTO. If no such option is
@@ -1247,6 +1249,15 @@ static void __init spectre_v1_apply_mitigation(void)
pr_info("%s\n", spectre_v1_strings[spectre_v1_mitigation]);
}

+#ifdef CONFIG_DYNAMIC_MITIGATIONS
+static void spectre_v1_reset_mitigation(void)
+{
+ setup_clear_cpu_cap(X86_FEATURE_FENCE_SWAPGS_USER);
+ setup_clear_cpu_cap(X86_FEATURE_FENCE_SWAPGS_KERNEL);
+ spectre_v1_mitigation = SPECTRE_V1_MITIGATION_AUTO;
+}
+#endif
+
static int __init nospectre_v1_cmdline(char *str)
{
spectre_v1_mitigation = SPECTRE_V1_MITIGATION_NONE;
@@ -3794,3 +3805,10 @@ void __warn_thunk(void)
{
WARN_ONCE(1, "Unpatched return thunk in use. This should not happen!\n");
}
+
+#ifdef CONFIG_DYNAMIC_MITIGATIONS
+void arch_cpu_reset_mitigations(void)
+{
+ spectre_v1_reset_mitigation();
+}
+#endif
--
2.34.1